How to get Blender 2.78c and Yafray to play nice...?
We can indeed enable or disable the addon only once per session (should be reported here: http://www.yafaray.org/development/bugtracker/yafaray).

Apart from that the Addon working fine on Windows (64 bit) :



First remove all the previous Add-ons and Versions of Yafaray, then download Yafaray 64bit from here, go to User Preferences, Addons, Install from File, select the zip file and enable the addon, done. 

If the error message comes up restart blender and enable/disable the addon.